1. Field of the Invention
This invention relates to a vortex flowmeter which converts a flow rate to be measured into an electrical signal using a vortex sensor to provide a vortex signal corresponding to the flow rate to be measured; and more particularly, to an improved vortex flowmeter which is capable of effectively eliminating noise imposed on a vortex sensor to deliver a more reliable vortex signal.
2. Discussion of the Prior Art
A conventional vortex flowmeter is depicted, for example, in FIGS. 1-4, wherein FIG. 1 shows a conventional vortex sensor comprising a pipe line 10 through which a fluid flows, and a cylindrical nozzle 11 attached perpendicular to pipe line 10. A columnar vortex generating member 12 of trapezoidal cross-section is inserted in pipe line 10 perpendicularly thereto with a spacing between member 12 and nozzle 11. One end of member 12 is secured to pipe line 10 by a screw 13 and the other end of member 12 is secured to nozzle 11 at a flange portion 14 by a screw or by welding. A cavity 15 is formed on the side of flange portion 14 of vortex generating member 12. Disposed in cavity 15 is a pair of piezoelectric elements 16 and 17 arranged vertically with a given spacing therebetween. Piezoelectric elements 16 and 17 are sealed and insulated from each other by a sealing member 18 made of glass or the like. Each piezoelectric element 16 and 17 has two electrodes of semi-circular shape disposed on each upper and lower side. The one piezoelectric piece sandwiched between the upper and lower electrodes on the left side of each piezoelectric element 16,17 is polarized in the opposite direction to the other piezoelectric piece sandwiched between the upper and lower electrodes on the right side, so that in response to stresses of the same direction, these piezoelectric pieces generate electric charges of opposite polarities on the upper and lower electrodes thereof.
Vortex signals from the thus configured vortex sensor are applied to the converting unit depicted in FIG. 2. In FIG. 2, electric charges Q.sub.V1 and Q.sub.V2 having a frequency corresponding to the vortex frequency of the vortex signals generated by piezoelectric elements 16 and 17 of the vortex sensor are applied to charge converters 19 and 20 where they are converted into AC voltage signals. The voltage signal of the charge converter 19 is added,in an adder 22, to a voltage signal obtained by passing the voltage signal of the charge converter 20 through a rheostat 21. The resultant sum output, after being low-pass filtered by a low-pass filter 23, is amplified by an amplifier 24 to a certain magnitude.
The output of amplifier 24 is applied to a Schmitt trigger 25 having a certain hysteresis width, so that the vortex signal having an amplitude which is larger than the hysteresis width is converted into a pulse signal whose frequency corresponds, one to one, to the vortex frequency.
This pulse signal, after having passed through a transformer 26, which provides DC insulation, is applied to a frequency voltage converter 27 so that it is converted into an analog voltage signal whose span is determined by a rheostat 28.
This voltage signal controls the base current of a transistor 31 via a DC amplifier 30 whose zero point is set by a rheostat 29, so that it is converted into a current output I.sub.L. This current output is transmitted, through the collector terminal and emitter teminal of the transistor, and the output terminals T.sub.1 and T.sub.2, to a receiving resistor R.sub.L of a receiving instrument having an external power source E.sub.S. As will be appreciated, a feedback resistor R.sub.f is inserted between the transistor 31 and the output terminal T.sub.2, and a feedback voltage E.sub.f generated across this feedback resistor R.sub.f is fed back to the input end of DC amplifier 30 so that the current output I.sub.L is controlled within the range of 4 to 20 mA corresponding to a voltage signal at the input.
A base part, of about 4 mA, out of the current output I.sub.L is used to create an internal power source for the converting unit. That is, a part of the current output is supplied through a constant current circuit 32 to a constant voltage circuit 33 which generates a reference voltage. This is used to generate a zero voltage across the rheostat 29. Furthermore, another part of the 4 mA current is supplied through a transistor 34 to a DC-AC converting circuit 35 where it is converted into an AC voltage. The thus converted AC voltage is supplied through a transformer 36 to an internal power source circuit 37. Circuit 37 creates internal voltages +V and -V necessary for operation of the converting unit.
The operation of the embodiment will now be described with reference to FIGS. 3(a)-3(c), and FIGS. 4. Upon flowing of a fluid, due to Karman vortices, vibration is generated on vortex generating member 12 of FIG. 1 in the directions of the double arrow F. Due to this vibration, a stress distribution and a counter stress distribution repeatedly appear, as shown in FIG. 3(a), on the vortex generating member 12. As a result, electric charges +Q and -Q repeatedly appear on each piezoelectric element 16,17, which correspond to a signal stress, as shown in FIG. 3(a), having the same frequency as the vortex frequency.
On the other hand, pipe line 10 involves pipe line vibration other than the above which will cause noise. The pipe line vibration is classified into components of three directions. (1) The drag direction which is in accord with the flowing of the fluid. (2) The buoyancy direction which is perpendicular to the flowing of the fluid. (3) The longitudinal direction of the vortex generating member. Among the foregoing, the stress distribution relating to the drag direction vibration becomes as shown in FIG. 3(b). That is, the positive and negative charges are cancelled out in each electrode to result in no noise charge. Furthermore, any charges due to the longitudinal direction vibration are cancelled out in the electrodes, as shown in FIG. 3(c), to result in no noise charge, similar to the case of the drag direction.
However, the vibration of the buoyancy direction F exhibits the same stress distribution as the signal stress to result in some noise charges. Accordingly, the following processing is performed for the purpose of eliminating such noise charges. Taking Q.sub.V1 and Q.sub.V2 as the respective charges of the piezoelectric elements 16,17, S.sub.1 and S.sub.2 as the signal components, and N.sub.1 and N.sub.2 as the noise components in the buoyancy direction, with the piezoelectric elements 16,17 being oppositely polarized, the Q.sub.V1 and Q.sub.V2 are given by the following expressions. EQU Q.sub.V1 =S.sub.1 +N.sub.1 EQU -Q.sub.V2 =-S.sub.2 -N.sub.2
provided that S.sub.1 and S.sub.2 and N.sub.1 and N.sub.2 have the same vector direction.
The relationship between the signal components and the noise components of the piezoelectric elements 16 and 17 is such as that shown in FIG. 4 which illustrates the relationship of the bending moment of the vortex generating member with respect to the noise and signal in the buoyancy direction. Thus, when the output of the charge converter 20 on the side of piezoelectric element 17 is multiplied by a factor of N.sub.1 /N.sub.2, using rheostat 21, and then added to the output of charge converter 19, the following is obtained so that the pipe line noise is eliminated. EQU Q.sub.V1 -Q.sub.V2 (N.sub.1 /N.sub.2)=S.sub.1 -S.sub.2 (N.sub.1 /N.sub.2)
In this manner, with a two element system, among the noises imposed on the vortex generating member, the noises in the drag direction and the longitudinal direction of the vortex generating member can be eliminated by paying attention to the polarities of the piezoelectric elements 16,17. Also, the noise in the buoyancy direction can be eliminated by paying attention to the balance between the piezoelectric elements 16,17.
Furthermore, other noises due to the presence of variations in property of the piezoelectric elements 16,17 and/or due to some defective adjustment for noise balance of the rheostat 21 can be eliminated by insertion of the low pass filter 23 in the converting unit.
However, although the foregoing conventional type of vortex flowmeter can exert a certain function of noise elimination where noise has only an ordinary magnitude, such a function is not sufficient if the magnitude of noise exceeds such ordinary magnitude.